1. Field of the Invention
The present invention relates to flat/vertical type vacuum tunneling transistors. More particularly, the present invention relates to flat/vertical type vacuum tunneling transistors which adopt a MOSFET-like flat or vertical structure so as to increase the degree of integration and can be operated at low operation voltages with high speeds.
2. Background of the Invention
In conventional semiconductor devices, the flow of current is conducted within semiconductors, so the moving velocity of electrons is affected by the crystal lattices or impurities therein. Recently, there have been developed semiconductor devices which comprise microtip type vacuum transistors. In such a vacuum transistor, electrons move in vacuum and thus, at non-limited speeds. Therefore, the vacuum transistors can be operated at ultra speeds. However, they suffer from disadvantages in that they are difficult to integrate on a mass scale and require relatively high voltages for their operation.
In order to better understand the background of the invention, a description will be given of conventional techniques in conjunction with the drawings.
With reference to FIG. 1, there is a basic structure of a MOSFET (n-channel). Typically ranging, in upper operation frequency (ft), from 20-30 GHz, Si FETs of this structure show a lamination of being applied only for the voltage-controlled oscillators (VCO) with several GHz, but not for the oscillators with extreme high frequency of several tens GHz. For SOI and GaAs FETs, they can be used at higher frequencies, but also suffer from disadvantages in that they are difficult to fabricate and expensive.
In detail, when a gate G and a drain D are applied with a voltage with a source S being grounded in the MOSFET structure of FIG. 1, a space charge region is formed below a gate G in a body B. If the voltage exceeds a threshold voltage, a channel P is formed beneath the gate G. The MOSFET in this state is said to be electrically conducted. For an n-channel MOS, electrons move along the channel from the source S to the drain D. The operation speed of the device is inversely proportional to the time which it takes for the electrons to move from the source S to the drain D. Thus, the shorter the channel is, the faster the electrons move. The frequency ft, indicating the speed of a device, at which the current gain is 1 upon grounding the drain, is approximately proportional to the mobility (xcexc) of electrons and inversely proportional to the square of a channel length.
Notice is taken of the mobility (xcexc) among the factors which determine the speed of a device. The mobility depends on the materials of the channel. For example, as long as the applied electric field is below 5xc3x97104 ([V/cm]), the mobility is about 5 times faster in GaAs than in Si. GaAs is therefore used to fabricate high speed transistors. Above all, however, if the lattice structure of the channel region is removed, that is, if the channel region is in a vacuum, the mobility does not act as a limitative factor any longer. Accordingly, it is expected that stronger electric fields could make faster the operation speed of the device which has a vacuum channel region.
With reference to FIG. 2, there is a conventional vacuum transistor with a microtip, which is modified from a field emission display (FED) structure. With a frequency (ft) of approximately 1 THz, this vacuum transistor can be applied for the extreme high frequency devices for which conventional FETs are unable to be applied.
As seen in this figure, electrons are emitted from a sharp-pointed cathode emitter under the influence of a high accelerating potential ranging from tens of volts of 100 volts or higher and are controlled by a phospher screen places over a common anode. The number of the electrons which move toward the anode are controlled by applying tens of volts to a gate. The reason why such high voltages are required to control and emit electrons is that the tip is apart from the gate at a relatively long distance. Together with the high anode and gate voltages, the difficulty in making such a microtip limits these vacuum transistor structures within particular applications, e.g. military use.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a novel flat/vertical type vacuum tunneling transistor, which allows a high degree of integration.
It is another object of the present invention to provide a novel flat/vertical type vacuum tunneling transistor, which can be operated at a very low voltage with high speeds.
The present invention adopts a MOS transistor-like flat or vertical structure, instead of a conventional microtip structure, so as to increase the integration degree, and recruits a low work function material to induce an tunneling effect under a lower voltage. In addition, the present invention is structured in such a way that electrons travel a vacuum free space, thereby realizing the high speed operation of devices. In conventional devices, such as Si and GaAs devices, electrons flow through the lattices consisting of Si or GaAs atoms. In result, the electrons collide with the atoms or impurities added, so they cannot freely move, but show limited mobility.
As a result of the intensive and thorough research on a novel vacuum transistor, repeated by the present inventors, a novel flat/vertical vacuum tunneling transistor which meets the above conditions, was developed and named xe2x80x9cVacuum Field Transistorxe2x80x9d (hereinafter referred to as xe2x80x9cVFTxe2x80x9d).
In accordance with an aspect of the present invention, there is provided a flat type vacuum field transistor, comprising a source and a drain, made of conductors, which stand at a predetermined distance apart on a thin channel insulator with a vacuum channel therebetween; a gate, made of a conductor, which is formed with a width below the source and the drain, the channel insulator functioning to insulate the gate from the source and the drain; and an insulating body, which serves as a base for propping up the channel insulator and the gate, wherein proper bias voltages are applied among the gate, the source and the drain to enable electrons to be field emitted from the source through the vacuum channel to the drain.
Preferable is the flat type vacuum field transistor comprising a low work function material at the contact regions between the source and the vacuum channel and between the drain and the vacuum channel.
Particularly preferable is a VFT structure in which each VFT device is installed in a trench consisting of septal walls in order that the electrons emitted from a source by a tunnel effect should not move through the vacuum free space toward neighboring drains.
In another aspect of the present invention, there is provided a vertical type vacuum field transistor, comprising; a conductive, continuous circumferential source with a void center, formed on a channel insulator; a conductive gate formed below the channel insulator, extending across the source; an insulating body for serving as a base to support the gate and the channel insulator; insulating walls which stand over the source, forming a closed vacuum channel; and a drain formed over the vacuum channel, wherein proper bias voltages are applied among the gate, the source and the drain to enable electrons to be filed emitted form the source through the vacuum channel to the drain.
Particularly preferable is the vertical type vacuum field transistor which further comprises a low work function material coated on the source.